The present invention provides methods for fabricating p-n junctions and to group II-VI semiconductor devices containing p-n junctions in which the p-n junctions contain concentration profiles for the p-type and n-type dopants that are controllable and independent of a diffusion profile.
P-N junctions have historically been fabricated by diffusion processes in which dopant species diffuse through a material from a region of high concentration to a region of low concentration. Fick's second law explains diffusion under non-steady state conditions:
                    C        s            -              C        x                            C        x            -              C        0              =      erf    (          x              2        ⁢                  Dt                      )                  where Cs=surface concentration of the dopant;                    C0=initial concentration of the dopant throughout the material;            Cx=concentration of the dopant at distance x from the surface at time t;            x=distance from surface;            D=diffusivity;            t=time;            erf=the “error function” as found in standard tables or graphs.                        
If C0=0, then
            C      x              C      s        =      erf    (          x              2        ⁢                  Dt                      )  
As D or t increases, the erf expression becomes smaller and Cx/Cs approaches 1. In other words, Cx approaches the surface concentration. The diffusivity D is a function of temperature. As temperature is increased, so does the diffusivity. The concentration profile of the dopant as a result of diffusion, referred to as the diffusion profile, may be determined from the foregoing equations.
Diffusion processes have been effective to prepare p-n junctions in semiconductor materials such as silicon, gallium arsenide, and gallium nitride. These materials are based upon 3 and 4 valent ions, which exist in a crystal lattice that readily permits diffusion. In contrast, zinc oxide and zinc sulfide are based upon divalent ions which exists in a crystal lattice that possesses unique challenges to the fabrication of doped semiconductor materials.
Zinc oxide (ZnO) and zinc sulfide (ZnS) are wide band gap semiconductors with potential for use in electrically excited devices such as light emitting devices (LEDs), laser diodes (LDs), field effect transistors (FETs), photodetectors operating in the ultraviolet and at blue wavelengths of the visible spectrum, and other similar devices. Zinc oxide and zinc sulfide are examples of group II-VI semiconductor compounds.
As used herein, group II-VI semiconductor compounds include group II elements selected from zinc, cadmium, the alkaline earth metals such as beryllium, magnesium calcium, strontium, and barium, and mixtures thereof, and group VI elements selected from oxygen, sulfur, selenium, tellurium, and mixtures thereof. The group II-VI semiconductor compounds may be doped with one or more p-type dopant. Such p-type dopants include, but are not limited to, nitrogen, phosphorus, arsenic, antimony, bismuth, copper, chalcogenides of the foregoing, and mixtures thereof. The group II-VI semiconductor compounds may be doped with one or more n-type dopants. Such n-type dopants include, but are not limited to, ions of Al, Ga, B, H, Yb and other rare earth elements, Y, Sc, and mixtures thereof.
Zinc oxide has several properties which make it an attractive for use as a group II-VI semiconductor. For instance, ZnO has a large exciton binding energy, which suggests that ZnO-based lasers should have efficient optical emission and detection. Zinc oxide drift mobility saturates at high fields and high values, which may lead to higher frequency device performance. The cost and ease of manufacture of zinc oxide is attractive when compared to other common semiconductor materials. Zinc oxide has excellent radiation-resistance (2 MeV at 1.2×1017 electrons/cm2), which is desirable for radiation hardened electronics. Zinc oxide has high thermal conductivity (0.54 W/cm·K). Zinc oxide has strong two-photon absorption with high damage thresholds, rendering it ideal for optical power limiting devices. Zinc oxide forms two stable polytypes: wurtzite and zincblende; however, polytypism is not as prevalent as with GaN, AlN, and SiC.
N-type zinc oxide semiconductor materials are known and relatively easy to prepare, such as ZnO doped with aluminum, gallium, or other known n-type dopants. P-type zinc oxide semiconductor materials are theoretically possible, but have been difficult to prepare. D. C. Look et al., “The Future of ZnO Light Emitters,” Phys. Stat. Sol., 2004, summarize data on p-type ZnO samples reported in the literature. The best reported ZnO samples have resistivity values of 0.5 ohm•cm (N/Ga dopants) and 0.6 ohm•cm (P dopant). Many of the reported p-type zinc oxide samples tend to be light, heat, oxygen, and moisture sensitive. Some convert to n-type material over time. Their stability has been questioned. Some of the more-stable p-type zinc oxide materials reported in the literature are prepared using complex and expensive fabrication processes, such as molecular beam epitaxy. No commercially viable p-type zinc oxide semiconductor materials are currently known.
Without being bound by theory, it is presently believed one possible explanation for the lack of p-type zinc oxide materials is because high temperature diffusion processes or other fabrication methods inhibit formation of desirable p-type zinc oxide compounds.
As mentioned above, temperature determines the effective limits for diffusion to be applicable in certain semiconductor systems. At low temperature the process is limited by slow diffusion, and when the temperature becomes sufficiently high for diffusion to occur, chemical reaction between the p-type dopant and oxygen forms stable gaseous species that make the p-type zinc oxide semiconductor structure unstable. Therefore, diffusion processes are not economically feasible for forming p-n junctions. Instead, a dynamic growth process should be used to fabricate p-n junctions.